与多线程、多进程相比，I/O复用最大的优势是系统开销小，系统不需要建立新的进程或者线程，也不必维护这些线程和进程。
代码示例：
#include <stdio.h> #include <unistd.h>#include <stdlib.h>#include <errno.h>#include <string.h>#include <sys/socket.h>#include <sys/types.h>#include <netinet/in.h>#include <arpa/inet.h>#include <sys/select.h>#define SERV_PORT 8080#define LIST 20//服务器最大接受连接#define MAX_FD 10//FD_SET支持描述符数量int main(int argc, char *argv[]){int sockfd;int err;int i;int connfd;int fd_all[MAX_FD]; //保存所有描述符，用于select调用后，判断哪个可读//下面两个备份原因是select调用后，会发生变化，再次调用select前，需要重新赋值fd_set fd_read; //FD_SET数据备份fd_set fd_select; //用于selectstruct timeval timeout;//超时时间备份struct timeval timeout_select; //用于selectstruct sockaddr_in serv_addr; //服务器地址struct sockaddr_in cli_addr; //客户端地址socklen_t serv_len;socklen_t cli_len;//超时时间设置timeout.tv_sec = 10;timeout.tv_usec = 0;//创建TCP套接字sockfd = socket(AF_INET, SOCK_STREAM, 0);if(sockfd < 0){perror("fail to socket");exit(1);}// 配置本地地址memset(&serv_addr, 0, sizeof(serv_addr));serv_addr.sin_family = AF_INET;// ipv4serv_addr.sin_port = htons(SERV_PORT);// 端口， 8080serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); // ipserv_len = sizeof(serv_addr);// 绑定err = bind(sockfd, (struct sockaddr *)&serv_addr, serv_len);if(err < 0){perror("fail to bind");exit(1);}// 监听err = listen(sockfd, LIST);if(err < 0){perror("fail to listen");exit(1);}//初始化fd_all数组memset(fd_all, -1, sizeof(fd_all));fd_all[0] = sockfd; //第一个为监听套接字FD_ZERO(&fd_read);// 清空FD_SET(sockfd, &fd_read); //将监听套接字加入fd_readint maxfd = fd_all[0]; //监听的最大套接字while(1){// 每次都需要重新赋值，fd_select，timeout_select每次都会变fd_select = fd_read;timeout_select = timeout;// 检测监听套接字是否可读，，没有可读，此函数会阻塞// 只要有客户连接，或断开连接，select()都会往下执行err = select(maxfd+1, &fd_select, NULL, NULL, NULL);//err = select(maxfd+1, &fd_select, NULL, NULL, (struct timeval *)&timeout_select);if(err < 0){perror("fail to select");exit(1);}if(err == 0){printf("timeout\n");}// 检测监听套接字是否可读if( FD_ISSET(sockfd, &fd_select) ){//可读，证明有新客户端连接服务器cli_len = sizeof(cli_addr);// 取出已经完成的连接connfd = accept(sockfd, (struct sockaddr *)&cli_addr, &cli_len);if(connfd < 0){perror("fail to accept");exit(1);}// 打印客户端的 ip 和端口char cli_ip[INET_ADDRSTRLEN] = {0};inet_ntop(AF_INET, &cli_addr.sin_addr, cli_ip, INET_ADDRSTRLEN);printf("———————————————-\n");printf("client ip=%s,port=%d\n", cli_ip,ntohs(cli_addr.sin_port));// 将新连接套接字加入 fd_all 及 fd_readfor(i=0; i < MAX_FD; i++){if(fd_all[i] != -1){continue;}else{fd_all[i] = connfd;printf("client fd_all[%d] join\n", i);break;}}FD_SET(connfd, &fd_read);if(maxfd < connfd){maxfd = connfd; //更新maxfd}}//从1开始查看连接套接字是否可读，因为上面已经处理过0（sockfd）for(i=1; i < maxfd; i++){if(FD_ISSET(fd_all[i], &fd_select)){printf("fd_all[%d] is ok\n", i);char buf[1024]={0}; //读写缓冲区int num = read(fd_all[i], buf, 1024);if(num > 0){//收到 客户端数据并打印printf("receive buf from client fd_all[%d] is: %s\n", i, buf);//回复客户端num = write(fd_all[i], buf, num);if(num < 0){perror("fail to write ");exit(1);}else{//printf("send reply\n");}}else if(0 == num){ // 客户端断开时//客户端退出，关闭套接字，并从监听集合清除printf("client:fd_all[%d] exit\n", i);FD_CLR(fd_all[i], &fd_read);close(fd_all[i]);fd_all[i] = -1;continue;}}else {//printf("no data\n");}}}return 0;}
运行结果：
源码下载：
世界会向那些有目标和远见的人让路(冯两努——香港着名推销商
